The invention relates to a method for solid body joining of a carrier body and a cover layer, in particular a method for anodic bonding, wherein the carrier body and the cover layer are connected areally. The invention further relates to a composite component comprising a carrier body and a cover layer, in particular an optical component, for example, a mirror. Applications of the invention are to be found in the manufacturing of composite components, particularly optical components, for example, mirrors for the purposes of optical imaging.
Anodic bonding is a generally known joining process for connecting planar surfaces of, for example, micromechanical components, semiconductor wafers or optical components. By way of example, in DE 10 2009 011 863, an optical mirror comprising a ceramic carrier body with a planar surface and a glass cover layer is disclosed, wherein the glass cover layer is connected by anodic bonding to the surface of the ceramic carrier body. The bond connection is created in that the glass cover layer is placed on the surface of the ceramic carrier body and pressed on in a central region in a locally restricted manner. When an electric potential and heat are applied to the bonding partners, ions, for example, potassium and sodium ions, migrate away from the boundary surface of the glass cover layer and the ceramic carrier body. By this means, a space-charge region is formed so that the mutually adjacent surfaces are attracted to one another until chemical compounds form between the ceramic and the oxygen in the glass cover layer. Starting from the central region, a bond front forms on all sides migrating to the edge of the mutually adjacent surfaces until the cover layer is firmly bound areally to the carrier body. Gaseous reaction products which can arise during the production of the bond connection at the bond front escape between the as yet unconnected surfaces toward their edge.
The conventional method has the advantage that mirrors are prepared which, due to the ceramic carrier body, have a high level of stiffness and mechanical stability and, due to the glass cover layer, have a smooth surface which can be made with optical quality. However, a disadvantage is that the bond front can migrate unevenly, so that gaps can form in the connection. A further disadvantage is that the conventional technique is restricted to anodic bonding of planar surfaces. However, there exists an interest in manufacturing mirrors with curved reflector surfaces, in particular mirrors with aspherical reflector surfaces, for example, for applications in space, for telescopes and in lithography.
Aspherical mirrors have conventionally been manufactured by mechanical processing (grinding, lapping and polishing) of a glass body. However, aspherical glass bodies have limited stability, in particular with large mirror diameters in the range of, for example, 10 cm to 20 cm, so that they are unsuitable for use under thermal or mechanical loading, particularly in space.
The following attempts at manufacturing curved mirrors with improved stability are known from practical applications. It has been proposed to apply a coating on a ceramic carrier body with a curved carrier body surface, for example a glass cover layer, by means of sputtering. However, this approach has the disadvantage that the thickness of a layer deposited by coating, for example, by sputtering or CVD is limited under practical process conditions. The low achievable thickness means that a subsequent mechanical processing of, for example, a glass, silicon or SiC cover layer which are applied by sputtering or CVD, for example, to construct an aspherical surface form, is possible to only a limited extent. Furthermore, the roughness of the ceramic carrier body is transferred to the surface of the cover layer and thus also influences the achievable optical quality.
A further proposal is based on connection by optical contact bonding, wherein surfaces are connected by molecular attraction forces. With the aid of optical contact bonding, joining is possible also on curved surfaces on provision of the required surface quality with regard to roughness and, in particular, accuracy of shape (DE 10200243 A1). However, the stability of the connection of parts joined by optical contact bonding is strongly dependent on the respective coefficients of expansion of the bonding partners and the associated permissible temperature of the surroundings. Furthermore, the connection by optical contact bonding is very susceptible to liquids and sealing is always necessary in order to prevent untimely detachment. These connections are also subject to mechanical instability (vibration drift). This can be reduced but not suppressed by means of plasma pre-treatment of the bonding partners since optical contact bonding does not lead to a positive substance bonded joint. Use in an environment with severe temperature variations and/or high mechanical loading (shock) is therefore precluded.
The stated problems occur not only during the manufacturing of composite components for optical tasks by means of anodic bonding. Other solid body joining processes such as glass-ceramic-metal soldering, diffusion welding or nanostructured bonding are restricted to the joining of planar surfaces as well.
It is an objective of the invention to provide an improved method for solid body joining of a carrier body and a cover layer, by which the disadvantages of conventional techniques can be overcome. The method should be usable, in particular, for areal, interruption-free connection of curved surfaces, for example, spherical or aspherical surfaces. It is a further objective of the invention to provide an improved composite component, in particular for optical purposes, with which disadvantages of conventional composite components are overcome. The composite component should be characterized, in particular, by an areal, interruption-free connection between mutually adjacent curved surfaces.
These objectives are achieved by means of a method for solid body joining, in particular for anodic bonding, and a composite component having the features of the independent claims. Advantageous embodiments and applications of the invention are disclosed in the dependent claims.